Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 91: What is the major storage form of fatty acids in the body?

A. Triacylglycerols (Correct Answer)
B. Cholesteryl esters
C. Cholesterol
D. Ketones

Explanation: **Explanation:** **Triacylglycerols (TAGs)**, also known as triglycerides, are the primary storage form of fatty acids in the human body. They consist of three fatty acid chains esterified to a single glycerol backbone. TAGs are stored predominantly in **adipose tissue** within specialized cells called adipocytes. **Why Triacylglycerols are the ideal storage form:** 1. **Energy Density:** They are highly reduced molecules, yielding approximately 9 kcal/g (compared to 4 kcal/g for carbohydrates). 2. **Hydrophobicity:** Unlike glycogen, TAGs are anhydrous (stored without water), allowing for a massive amount of energy to be packed into a small volume without increasing the cell's osmotic pressure. **Analysis of Incorrect Options:** * **B. Cholesteryl esters:** These are the storage form of cholesterol within cells and are found in the core of lipoproteins (like LDL), but they are not the primary storage form for fatty acids. * **C. Cholesterol:** This is a structural component of cell membranes and a precursor for steroid hormones and bile acids; it is not used as an energy storage fuel. * **D. Ketones:** These (acetoacetate, β-hydroxybutyrate) are water-soluble energy alternatives produced by the liver during starvation or prolonged exercise, but they are transport forms, not storage forms. **High-Yield Clinical Pearls for NEET-PG:** * **Hormone-Sensitive Lipase (HSL):** This is the rate-limiting enzyme for mobilizing fatty acids from adipose tissue (stimulated by Glucagon/Epinephrine). * **Brown Adipose Tissue:** Contains **Thermogenin (UCP-1)**, which uncouples the electron transport chain to generate heat instead of ATP. * **Steatosis:** Excessive accumulation of TAGs in non-adipose tissues (like the liver) leads to fatty liver disease.

Question 92: Bile acids are synthesized from which precursor molecule?

A. Arachidonic acid
B. Acetyl CoA (Correct Answer)
C. Linolenic acid
D. Linoleic acid

Explanation: **Explanation:** The synthesis of bile acids is the primary pathway for the excretion of cholesterol from the body. To understand why **Acetyl CoA** is the correct answer, one must trace the biosynthetic hierarchy: 1. **Cholesterol Synthesis:** All carbon atoms in cholesterol are derived from **Acetyl CoA** via the HMG-CoA reductase pathway (the rate-limiting step). 2. **Bile Acid Synthesis:** Cholesterol is the direct precursor for bile acids. In the liver, cholesterol is converted into primary bile acids (Cholic acid and Chenodeoxycholic acid) by the rate-limiting enzyme **7-alpha-hydroxylase**. 3. **Conclusion:** Since cholesterol is synthesized entirely from Acetyl CoA, Acetyl CoA serves as the fundamental building block for bile acids. **Analysis of Incorrect Options:** * **Arachidonic acid (Option A):** A 20-carbon polyunsaturated fatty acid (PUFA) that serves as a precursor for eicosanoids (prostaglandins, leukotrienes, and thromboxanes), not steroids or bile acids. * **Linolenic (Option C) and Linoleic acid (Option D):** These are essential fatty acids. While they are involved in membrane structure and signaling, they are not precursors for the steroid nucleus found in bile acids. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** 7-alpha-hydroxylase (inhibited by bile acids, stimulated by cholesterol). * **Primary vs. Secondary:** Primary bile acids are made in the liver; **Secondary bile acids** (Deoxycholic and Lithocholic acid) are formed by bacterial action in the gut. * **Conjugation:** Bile acids are conjugated with **Glycine or Taurine** in the liver to increase their solubility (forming bile salts). * **Enterohepatic Circulation:** Approximately 95% of bile salts are reabsorbed in the **terminal ileum**.

Question 93: Fatty acid synthesis and degradation are similar in that both processes:

A. Take place in the mitochondrion
B. Use acyl CoA-thioesters (Correct Answer)
C. Use NADPH or NADP as a cofactor
D. Have malonyl CoA as an intermediate

Explanation: **Explanation:** The correct answer is **B. Use acyl CoA-thioesters**. Both fatty acid synthesis (lipogenesis) and degradation (beta-oxidation) involve the formation of high-energy thioester bonds between the fatty acid chain and a carrier molecule. In degradation, the intermediates are linked to **Coenzyme A (CoA)**. In synthesis, while the intermediates are primarily bound to the **Acyl Carrier Protein (ACP)**, the initial priming steps and the final release involve CoA-thioesters. **Why other options are incorrect:** * **A. Take place in the mitochondrion:** Fatty acid **degradation** occurs in the mitochondria, but **synthesis** occurs primarily in the **cytosol**. (Note: Elongation can occur in the ER/mitochondria). * **C. Use NADPH or NADP as a cofactor:** Synthesis uses **NADPH** (as a reducing agent), whereas degradation uses **NAD+ and FAD** (as oxidizing agents). * **D. Have malonyl CoA as an intermediate:** Malonyl CoA is a key intermediate and regulator of **synthesis**; it does not exist as an intermediate in the degradation pathway. In fact, Malonyl CoA inhibits *Carnitine Palmitoyltransferase I (CPT-1)* to prevent simultaneous synthesis and breakdown. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzymes:** Acetyl CoA Carboxylase (Synthesis) vs. CPT-1 (Degradation/Transport). * **The "Citrate Shuttle":** Citrate carries acetyl groups from the mitochondria to the cytosol for synthesis. * **The "Carnitine Shuttle":** Carnitine transports long-chain fatty acids into the mitochondria for degradation. * **Energy Source:** NADPH for synthesis is mainly supplied by the **HMP Shunt** (Pentose Phosphate Pathway).

Question 94: Alcohol intake primarily affects which of the following lipid fractions?

A. Triglycerides (TG)
B. Low-density lipoprotein (LDL)
C. Very low-density lipoprotein (VLDL)
D. High-density lipoprotein (HDL) (Correct Answer)

Explanation: **Explanation:** The primary and most consistent effect of moderate alcohol consumption on the lipid profile is an **increase in High-Density Lipoprotein (HDL) levels**. **Why HDL is the correct answer:** Alcohol increases HDL levels through two main mechanisms: 1. **Increased Synthesis:** Alcohol stimulates the hepatic synthesis of Apolipoprotein A-I and A-II, the primary protein components of HDL. 2. **Decreased Clearance:** It inhibits the activity of **Cholesteryl Ester Transfer Protein (CETP)**. Since CETP normally transfers cholesterol from HDL to VLDL/LDL, its inhibition leads to higher cholesterol retention within the HDL fraction, specifically increasing HDL2 and HDL3 subfractions. This is often cited as a reason for the "cardioprotective" effect of moderate alcohol. **Analysis of Incorrect Options:** * **Triglycerides (TG) & VLDL:** While chronic or excessive alcohol intake (binge drinking) significantly increases TG and VLDL levels by increasing fatty acid synthesis and impairing oxidation, these changes are more variable and dose-dependent compared to the consistent rise in HDL seen with moderate intake. * **LDL:** Alcohol typically has a neutral or slightly lowering effect on LDL levels. It does not primarily elevate LDL; in fact, high LDL is more closely associated with saturated fat intake and genetic factors. **High-Yield Clinical Pearls for NEET-PG:** * **Alcoholic Hyperlipidemia:** The most common pattern seen in heavy drinkers is **Type IV Hyperlipoproteinemia** (elevated VLDL/TG). * **NADH/NAD+ Ratio:** Alcohol metabolism increases the NADH/NAD+ ratio, which shifts the balance toward glycerol-3-phosphate, providing the backbone for TG synthesis (leading to fatty liver). * **Cardiovascular Effect:** The increase in HDL is the biochemical basis for the "J-shaped curve" relationship between alcohol and coronary heart disease.

Question 95: What is the biochemical defect in Zellweger syndrome?

A. Peroxisomal biogenesis disorder (Correct Answer)
B. Lysosomal targeting disorder
C. Defect in glycosylation of proteins
D. Trisomy 21

Explanation: **Biochemical Explanation:** Zellweger syndrome (Cerebro-hepato-renal syndrome) is the most severe form of the **Peroxisomal Biogenesis Disorders (PBD)**. It is caused by mutations in the **PEX genes**, which encode proteins called **peroxins**. These proteins are essential for the assembly of peroxisomes and the import of enzymes into the peroxisomal matrix. Without functional peroxisomes, the cell cannot perform critical metabolic processes, most notably the **beta-oxidation of Very Long Chain Fatty Acids (VLCFA)** and the synthesis of plasmalogens (essential for myelin). This leads to the accumulation of VLCFAs in the blood and tissues, causing severe neurological and multi-organ dysfunction. **Analysis of Incorrect Options:** * **Option B (Lysosomal targeting disorder):** This refers to **I-cell disease**, where a defect in N-acetylglucosaminyl-1-phosphotransferase prevents the addition of the Mannose-6-Phosphate tag, causing enzymes to be secreted extracellularly rather than sent to lysosomes. * **Option C (Defect in glycosylation):** This describes **Congenital Disorders of Glycosylation (CDG)**, which typically involve defects in the synthesis of N-linked oligosaccharide chains. * **Option D (Trisomy 21):** This is **Down Syndrome**, a chromosomal numerical abnormality, not a primary metabolic or organelle biogenesis defect. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Hypotonia (floppy baby), seizures, and distinctive facial features (high forehead, widened fontanelles). * **Diagnostic Marker:** Elevated levels of **VLCFA** (e.g., C26:0 and C26:1) in the plasma. * **Organ Involvement:** Hepatomegaly, jaundice, and renal cysts are common. * **Prognosis:** Usually fatal within the first year of life.

Question 96: Where is HDL synthesized and secreted from?

A. Liver (Correct Answer)
B. Peripheral tissue
C. Adipose tissue
D. Muscle

Explanation: **Explanation:** High-Density Lipoprotein (HDL) is primarily synthesized and secreted by the **Liver** and, to a lesser extent, the **Small Intestine**. It is initially secreted as "nascent HDL," which are discoidal particles composed mainly of phospholipids (lecithin) and apolipoproteins (Apo A-I, A-II, C, and E). **Why the other options are incorrect:** * **Peripheral Tissues:** These tissues do not synthesize HDL. Instead, they are the *source* of excess cholesterol. HDL acts as a scavenger, picking up cholesterol from peripheral tissues via the ABCA1 transporter to transport it back to the liver (Reverse Cholesterol Transport). * **Adipose Tissue:** This is the primary site for triglyceride storage, not lipoprotein synthesis. It interacts with VLDL and Chylomicrons via Lipoprotein Lipase (LPL) but does not produce HDL. * **Muscle:** Similar to adipose tissue, muscle is a site for fatty acid oxidation and utilizes lipids delivered by other lipoproteins but lacks the enzymatic machinery to synthesize HDL particles. **High-Yield NEET-PG Pearls:** 1. **Reverse Cholesterol Transport:** This is the most important function of HDL, making it "Good Cholesterol" because it protects against atherosclerosis. 2. **Apo A-I:** This is the major apolipoprotein associated with HDL and serves as an activator for the enzyme **LCAT** (Lecithin-Cholesterol Acyltransferase). 3. **LCAT Function:** It converts free cholesterol into cholesterol esters within the HDL particle, transforming discoidal nascent HDL into spherical mature HDL (HDL3 and HDL2). 4. **CETP (Cholesterol Ester Transfer Protein):** Facilitates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL.

Question 97: Which of the following statements regarding ketone bodies is true?

A. Acetoacetate is the most common ketone body present in blood.
B. Beta-hydroxybutyrate is the first ketone body to be synthesized.
C. Thiophorase is absent in the liver. (Correct Answer)
D. Muscle cannot utilize ketone bodies.

Explanation: **Explanation:** Ketone bodies (Acetoacetate, $\beta$-hydroxybutyrate, and Acetone) are synthesized in the liver mitochondria during states of low glucose availability (starvation, diabetes mellitus). **Why Option C is Correct:** The liver is the primary site of **ketogenesis** (production), but it cannot perform **ketolysis** (utilization). This is because the liver lacks the enzyme **Thiophorase** (also known as succinyl-CoA:3-ketoacid CoA-transferase). This enzyme is essential for activating acetoacetate into acetoacetyl-CoA. The absence of Thiophorase ensures that the liver does not consume the fuel it produces, allowing ketone bodies to be exported to extrahepatic tissues. **Analysis of Incorrect Options:** * **Option A:** **$\beta$-hydroxybutyrate** is the most abundant ketone body in the blood, not acetoacetate. The ratio of $\beta$-HB to acetoacetate typically increases during ketosis. * **Option B:** **Acetoacetate** is the first ketone body synthesized in the HMG-CoA lyase reaction. $\beta$-hydroxybutyrate and acetone are subsequently derived from it. * **Option D:** Peripheral tissues, including **skeletal muscle, cardiac muscle, and the renal cortex**, are the primary consumers of ketone bodies. During prolonged starvation, even the brain adapts to utilize them. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Acetone:** A non-metabolizable side product excreted via lungs, causing the characteristic "fruity odor" in Diabetic Ketoacidosis (DKA). * **RBCs:** Cannot utilize ketone bodies because they lack mitochondria. * **Ketone Body Detection:** The **Rothera’s Test** detects Acetoacetate and Acetone, but **not** $\beta$-hydroxybutyrate.

Question 98: Regarding the synthesis of triacylglycerol in adipose tissue, all of the following are true except?

A. Synthesis from dihydroxyacetone phosphate
B. Enzyme glycerol kinase plays an important role (Correct Answer)
C. Enzyme glycerol-3-phosphate dehydrogenase plays an important role
D. Phosphatidate is hydrolyzed

Explanation: ### Explanation The synthesis of triacylglycerol (TAG) in adipose tissue requires **Glycerol-3-Phosphate** as the initial substrate. However, the metabolic pathways available to generate this substrate differ significantly between the liver and adipose tissue. **Why Option B is the Correct Answer (The "Except"):** Adipose tissue **lacks the enzyme Glycerol Kinase**. Therefore, it cannot phosphorylate free glycerol to form glycerol-3-phosphate. In contrast, the liver possesses high levels of glycerol kinase, allowing it to utilize glycerol directly. This is a classic "high-yield" distinction in biochemistry. **Analysis of Other Options:** * **Option A & C:** In adipose tissue, glycerol-3-phosphate must be derived from glucose via glycolysis. The intermediate **Dihydroxyacetone phosphate (DHAP)** is reduced to glycerol-3-phosphate by the enzyme **Glycerol-3-phosphate dehydrogenase** (utilizing NADH). Thus, both the substrate (DHAP) and the enzyme are essential in adipocytes. * **Option D:** During TAG synthesis (the Kennedy pathway), two fatty acyl-CoAs are added to glycerol-3-phosphate to form **Phosphatidate** (Phosphatidic acid). This phosphatidate must be **hydrolyzed** by phosphatidate phosphatase to form 1,2-diacylglycerol (DAG) before the final fatty acid can be added. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Glucose Dependency:** Because adipose tissue lacks glycerol kinase, TAG synthesis is entirely dependent on glucose uptake. In states of low insulin (like Diabetes Mellitus), glucose entry into adipocytes via **GLUT-4** is reduced, leading to decreased TAG synthesis and increased lipolysis. 2. **The Liver Advantage:** The liver can synthesize TAG during both the well-fed state (from glucose) and the fasting state (by recycling glycerol released from lipolysis), thanks to the presence of glycerol kinase. 3. **Rate-limiting step:** The activation of fatty acids by **Thiokinase** (Acyl-CoA synthetase) is a prerequisite for their incorporation into TAG.

Question 99: The carnitine shuttle system is primarily involved in which metabolic process?

A. Fatty acid synthesis
B. Fatty acid oxidation (Correct Answer)
C. Cholesterol synthesis
D. Bile acid synthesis

Explanation: **Explanation:** The **carnitine shuttle** is the rate-limiting step for **Beta-oxidation (Fatty acid oxidation)**. Long-chain fatty acids (LCFA) are activated in the cytosol to Fatty Acyl-CoA but cannot cross the impermeable inner mitochondrial membrane. The shuttle system, consisting of **Carnitine Palmitoyltransferase-I (CPT-I)**, Carnitine-acylcarnitine translocase, and CPT-II, transports these fatty acids into the mitochondrial matrix where oxidation occurs to produce energy. **Analysis of Options:** * **Option B (Correct):** Beta-oxidation occurs in the mitochondria. CPT-I (the key regulatory enzyme) converts Acyl-CoA to Acyl-carnitine, allowing entry into the matrix. * **Option A:** Fatty acid synthesis occurs in the **cytosol**. It uses the **Citrate shuttle** to move acetyl groups out of the mitochondria, not the carnitine shuttle. * **Option C & D:** Cholesterol and bile acid synthesis primarily occur in the cytosol and endoplasmic reticulum; they do not require carnitine-mediated transport. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** **Malonyl-CoA** (the first intermediate of fatty acid synthesis) inhibits CPT-I. This prevents a "futile cycle" by ensuring synthesis and oxidation do not happen simultaneously. * **Systemic Carnitine Deficiency:** Presents with **hypoketotic hypoglycemia** during fasting, as the liver cannot oxidize fats to produce energy or ketone bodies. * **Location:** CPT-I is located on the outer mitochondrial membrane; CPT-II is on the inner mitochondrial membrane.

Question 100: Zellweger syndrome is a disorder of which organelle?

A. Mitochondria
B. Lysosome
C. Golgi complex
D. Peroxisome (Correct Answer)

Explanation: **Explanation:** **Zellweger syndrome** (also known as cerebrohepatorenal syndrome) is the most severe form of the **Perisoxome Biogenesis Disorders (PBD)**. It is caused by mutations in the **PEX genes**, which are essential for the normal assembly and functioning of peroxisomes. Without functional peroxisomes, the body cannot perform **alpha-oxidation** or the **beta-oxidation of Very Long Chain Fatty Acids (VLCFA)**. This leads to the toxic accumulation of VLCFAs in the blood and tissues, particularly affecting the brain, liver, and kidneys. **Why other options are incorrect:** * **Mitochondria:** While mitochondria are the primary site for the beta-oxidation of short, medium, and long-chain fatty acids, they cannot process VLCFAs (chains >22 carbons). Mitochondrial disorders typically present with myopathy and lactic acidosis (e.g., MELAS). * **Lysosome:** Lysosomal storage disorders (e.g., Gaucher or Tay-Sachs) involve the deficiency of hydrolytic enzymes, leading to the accumulation of sphingolipids or glycosaminoglycans, rather than VLCFAs. * **Golgi complex:** The Golgi is responsible for protein modification and packaging. Disorders of the Golgi usually manifest as Congenital Disorders of Glycosylation (CDG). **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Hypotonia (floppy baby), neonatal seizures, and dysmorphic facial features (high forehead, wide fontanelles). * **Biochemical Marker:** Elevated levels of **VLCFA** in the plasma is the diagnostic hallmark. * **Associated Findings:** Hepatomegaly, jaundice, and **chondrodysplasia punctata** (stippled epiphyses on X-ray). * **Other Peroxisomal Disorders:** X-linked Adrenoleukodystrophy (defect in ABCD1 transporter) and Refsum disease (defect in alpha-oxidation).

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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